Electric motor fuel pumps have been widely used to supply the fuel demand of an operating engine, such as in automotive applications. These pumps may be mounted directly within a fuel supply tank and have an inlet through which fuel from the tank is drawn into the fuel pump and an outlet through which fuel is discharged under pressure for delivery to the engine. The electric motor in the pump typically includes a rotor mounted for rotation about its axis in a housing in response to application of electrical power to the motor. In so-called turbine-type fuel pumps, the motor drives an impeller for rotation to increase the pressure of fuel and deliver it to the engine. One example of a turbine-type fuel pump is illustrated in U.S. Pat. No. 5,257,916.
In general, it may be desirable to reduce leakage in the pump assembly to improve the efficiency of the pump. However, reducing leakage generally requires manufacturing the pump to tighter or smaller tolerances and that leads to increased costs and difficulties in manufacturing the pump assembly. For example, a typical pump assembly has an impeller with opposed generally planar faces disposed between two plates each having a generally planar face adjacent to the impeller. To reduce leakage between the impeller and the plates, the clearance between their adjacent faces must be made small. However, reducing the clearance between the plates and the impeller can unduly increase the friction between them and thereby affect the performance of the fuel pump. Accordingly, various methods have been employed to control the relative spacing between the impeller and the plates including lapping of one or more of the planar surfaces to insure compliance with strict tolerances, and grinding of the periphery of the impeller or other adjacent surfaces to insure their size and shape are within the closely held tolerances.
An additional factor to be considered in the manufacture and assembly of the fuel pump assembly is that the pressure of the fuel between the inlet and outlet of the pumping assembly is varied. At the inlet, the pressure may be at or below atmospheric pressure, while at the outlet the pressure may be substantially above atmospheric pressure and, for example, on the order of 40-80 psi or higher. Accordingly, the forces acting on the impeller and the rest of the pumping assembly vary greatly as a function of the pressure of fuel in the various regions of the pumping assembly. The varied forces across the impeller and the pumping assembly as a whole produce side loading and torque on a shaft that drives the impeller as well as a tendency to displace the pumping elements and adjacent plates thereby increasing friction between them. These conditions also occur in so-called two stage fuel pumps that have two pumping elements arranged in series.